284 THE EVOLUTION OF SEX. 



numerical strength of the species or in its increase. The 

 common tapeworm produces myriads of embryos, but these 

 have only one chance in eighty-five millions (it is said) of 

 succeeding. j\Iany common and numerous animals repro- 

 duce very slowly. That some species are on the increase, e.g., 

 bacteria, under the unprecedentedly favourable conditions which 

 our recent " industrial progress '"' affords, while other species 

 are on the decrease, e.g., many birds, is certain; but the rate 

 of reproduction is not a direct condition in either case. 



§ 2. History of Discussion on Rate of Reproduction. — In this, 

 as in not a few other cases, the biologist is profoundly indebted 

 to the student of social questions, for no adequate attention was 

 paid to the laws of multiplication before the appearance of the 

 epoch-making "theory of population" of ]\Ialthus, nor is it yet 

 possible or profitable to isolate the human question from the 

 general one. ]\Ialthus's fundamental proposition is indeed 

 usually softened from its earliest form — that population tends 

 to increase in geometrical, subsistence only in arithmetical 

 ratio— into the simple statement that population tends to out- 

 run subsistence, but has none the less served as a base of 

 weighty deductions for both the naturalist and the economist. 

 From Darwin's standpoint, the " positive checks "' to population 

 (disease, starvation, war, infanticide), and the "prudential'" 

 (moral or birth-restricting) checks, come to be viewed as special 

 forms of natural or artificial selection, while the fundamental 

 induction has been extended throughout nature as the essential 

 condition of the struggle for existence. After long dispute, the 

 induction of ]\Ialthus gained acceptance, followed by wide 

 deductive use and abuse, among economists. Yet, fundament- 

 ally important as the subject thus is to naturalist and economist 

 alike, the former has not as yet effected any thorough investi- 

 gation of the conditions of multiplication, or even usually 

 incorporated the keen analysis which we owe to Spencer, while 

 the economic theorist or disputant frequently still employs the 

 doctrine even in its pre-Darwinian form. It is thus doubly 

 needful to summarise, as briefly as may be, Spencer's elaborate 

 statement of the laws of multiplication. 



§ 3. Siu7imary of Spencer' s Analysis. — Different species exhibit difterent 

 degrees of fertility, which have become established in process of evolution 

 like the organisms themselves. To understand this particular adaptation 

 of function to conditions of existence, of organism to en\'ironment, we may 

 analyse these into their respective factors. It is evident that in the environ- 

 ment of any species there are many conditions with which its individuals 



